#107 Optimised bit clear with masks

[Oliver Sedlacek]

C does not really have any bit operations, which is a
nuisance if the target microprocessor has bit
manipulation instructions. To set or clear a bit in a
byte the programmer may choose to go with the portable
C code or drop down into non-portable assembler. Eg.

char abyte;
const char bitOfInterestMask = 0x01;

/* set the bit */
abyte |= bitOfInterestMask;
/* clear the bit */
abyte &= ~bitOfInterestMask;

I noticed on the HC08 port that the set operation is
efficiently optimised to the bset instruction but the
clear operation isn't optimised to the bclr instruction.

[Erik Petrich]

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The code generator already supports using bclr this way.
Could you post a compilable example in which |= reduces to
bset, but &= does not reduce to bclr? I tried your code
snippet in a number of different contexts and found that
either both bit set and clear would be optimized or neither
would.

[Oliver Sedlacek]

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I've uploaded the C source of an example. The code snippet
of significance is lines 257 and 258,
if(hectoRPM.b[U16_MSB] >
miscCfg.stc.REV_LIMITER) PORTB |= MskPBRevLmt;
else PORTB &= ~MskPBRevLmt;
This was compiled with version 2.5.0 using
sdcc -c -mhco8 measure.c

[Oliver Sedlacek]

Example C source

[Frieder Ferlemann]

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the mcs51 port doesn't always use bitset/bitclear operations
either:

#include <8052.h>
void main(void)
{
/* P0 is a bit-addressable SFR (an SFR
which is dividable by 8 without rest) */
P0 |= 0x01;
}